Timer Wheel, Jiffies and HZ (or, the way it was)

The original kernel timer system (called the "timer wheel) was based on
incrementing a kernel-internal value (jiffies) every timer
interrupt. The timer interrupt becomes the default scheduling
quamtum, and all other timers are based on jiffies. The
timer interrupt rate (and jiffy increment rate) is defined
by a compile-time constant called HZ. Different platforms
use different values for HZ. Historically, the kernel used
100 as the value for HZ, yielding a jiffy interval of 10 ms.
With 2.4, the HZ value for i386 was changed to 1000, yeilding
a jiffy interval of 1 ms. Recently (2.6.13) the kernel
changed HZ for i386 to 250. (1000 was deemed too high).

Ingo Molnar's explanation of timer wheel performance

Ingo Molnar did an in-depth explanation about the performance of
the current "timer wheel" implementation of timers. This was
part of a series of messages trying to justify the addition
of ktimers (which have different characteristics).

ktimers

Update: ktimers have been replaced by hrtimer framework also by Thomas Gleixner, only using a set of functions and datastructures in linux/ktime.h.

Material needs rework

A bunch of material in this section needs to be created or expanded to take into account the hrtimer system by Thomas Gleixner.

clock events

clock sources

Timer information

There are two /proc files that are very useful for gathering information about timers on your system.

/proc/timer_list

/proc/timer_list has information about the currently configured clocks and timers on the system.
This is useful for debugging the current status of the timer system (especially while you are developing
clockevent and clocksource support for your platform.)

You can tell if high resolution is configured for you machine by looking at a few different things:

For standard resolution (at jiffy resolution), a clock will have a value for it's '.resolution' field
equal to the period of a jiffy. For embedded machines, where HZ is typically 100, this will be
10 milliseconds, or 10000000 (ten million) nanoseconds.

Also for standard resolution, the Clock Event Device will have an event handler of "tick_handle_periodic".

For high resolution, the resolution of the clock will be listed as 1 nanosecond
(which is ridiculous, but serves as an indicator of essentially arbitrary precision.)
Also, the Clock Event Device will have an event handler of "hrtimer_interrupt".

[need more info here - and this should probably be written up and put in Documentation/filesystems/proc.txt]

/proc/timer_stats

/proc/timer_stats is a file in the /proc pseudo file system which allows you to see information
about the routines that are requesting timers of the Linux kernel. By cat'ing this file,
you can see which routines are using lots of timers, and how frequently they are requesting them.
This can be of interest to see

To use /proc/timer_stats, configure the kernel with support for the feature.
That is, set CONFIG_TIMER_STATS=y in your .config.
This is on the Kernel Hacking menu, with the prompt:
"Collect kernel timers statistics"

Compile and install your kernel, and reboot your machine.

To activate the collection of stats (and reset the counters),
do "echo 1 >/proc/timer_stats"

To stop collecting stats, do "echo 0 >/proc/timer_stats"

You can dump the statistics either while the collection system
is running or stopped.
To dump the stats, use 'cat /proc/timer_stats'. This shows the average events/sec at
the end as well so you get a rough idea of system activity.

/proc/timer_stats fields (for version 0.1 of the format) are:

<count>, <pid> <command> <start_func> (<expire_func>)

Dynamic ticks

Tickless kernel, dynamic ticks or NO_HZ is a config option that enables a kernel to run without a regular timer tick. The timer tick is a timer interrupt that is usually generated HZ times per second, with the value of HZ being set at compile time and varying between around 100 to 1500. Running without a timer tick means the kernel does less work when idle and can potentially save power because it does not have to wake up regularly just to service the timer. The configuration option is CONFIG_NO_HZ and is set by Tickless System (Dynamic Ticks), on the Kernel Features configuration menu.

Powertop

Powertop is a tool that parses the /proc/timer_stats output and gives a picture of what is causing wakeups on your system. Minimizing these wakeups should allow you to decrease power consumption in your device. Powertop was originally written for the x86 architecture but also works for embedded processors. However, in order to get a clean display from it, you will need an ncurses lib with wide character support.

High Resolution Timers

Old timer wheel/jiffy replacement proposals

Jun Sun's "tock" proposal

This systems replaces jiffies and xtime with tocks (arch-dependent), mtime (monotonic time) and wtime (wall time),
and proposes a strategy for migrating to that.

John Stultz

In 2005, John Stultz proposed changes to the timers to use a 64-bit nanosecond value as the base.
He did a presentation and BOF at OLS 2005. (It should be available online)

Timer Tick Thread - LKML July 2005

There was a very long thread about timers, jiffies, and related subjects
in July of 2005 on the kernel mailing list.

The title was: "Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt"

Linus said jiffies is not going away

- still need 32-bit counter, shouldn't be real-time value (too much overhead to calculate)
- high-res timers shouldn't be sub-HZ, but instead, HZ should be high and timer tick should not be 1:1 with HZ
- in other words, have HZ be high (like 2K), have the timer interrupt fire off at some lower frequency,
and increment jiffies by more than one on each interrupt.
- rationale for this is to keep a single sub-system

Arjan had good points about coalescing low-res timers

- 3 use cases:
- low res timeouts
- high res timer for periodic absolute wakeup (wake up every 10 ms, whether last one was late or nt
- high res timer for periodic relative wakeup (wake up 10 ms from now)